The invention relates to optical signal equalization and reshaping and more particularly to dynamic gain equalization and reshaping of optical signals in WDM (wavelength division multiplexed) systems.
In a long haul optical network, in order to maintain operating signal power over large distances of fiber span, optical signals are amplified at various amplifier sites along the fiber span. In WDM systems, multiple channels of different wavelengths can be used to transmit multiple streams of information along a single optical waveguide. This multichannel transmission is subject to degradation and attenuation over long fiber spans, and periodically retransmission can be used to restore signal quality and strength. Amplification is used to reduce costs incurred by multichannel retransmission. Typically this is achieved purely in the optical domain through EDFAs (Erbium doped fiber amplifiers) which amplify the light of the various channels of the WDM system passing through the amplifier. Despite the tremendous recent success in the deployment of EDFAs in WDM networks, EDFA amplification exhibits wavelength dependency which has become a significant problem in optical systems design.
Spectral power uniformity is crucial to maintain operational stability in a WDM optical network. Spectral variations in the gain of an amplifier distort the spectrum of a multichannel transmitter. Since EDFAs saturate in a largely homogeneous manner, strong and weak channels going into an amplifier compete for pumped Erbium ions. The result is that some channels will grow at the expense of the others, an effect which is even more prominent when optical amplifiers are cascaded. In fact, the ratio of the power in the high-gain channels to that of the low-gain channels will increase exponentially with the number of amplifiers in the chain, potentially causing significant channel-to-channel variations in signal-to-noise ratio in long span systems. Since optical nonlinearities limit the maximum power that can be put into a particular channel, large differences in channel powers make it difficult to keep the weaker channels above the receiver power threshold. Furthermore, large power imbalances can enhance cross-talk from strong to weak channels and may cause receiver dynamic range problems. These problems can be rectified if the relative channel power can be equalized. With the cascading of multiple non-uniform gain of EDFAs for extended geographic reach and channel flexible add/drop capability, dynamic gain equalization is urgently needed.
In long haul systems, gain equalization is typically implemented on a per amplifier basis. To avoid large power imbalances caused by cascaded EDFAs, gain equalization usually occurs in conjunction with amplification, occurring after each amplification stage so that each time a signal is amplified, it is also equalized. Typically, dynamic gain equalization is employed only every few amplifiers while at the remaining amplifiers, signals undergo static gain equalization. One current static equalization solution is to use a filter with a frequency spectrum which matches the inverse of the EDFA frequency profile. Since the EDFA frequency profile changes with the amplitude of the incident signal, a dynamic solution really is required to compensate for the EDFA power imbalances. The current general approach to dynamic gain equalization is to demultiplex the WDM multichannel signal, equalize the channels by attenuating them individually, and then multiplexing them back into a WDM multichannel signal.
Current implementations of dynamic gain equalization using this approach involve complex controlling electronics utilizing a closed loop design which monitors signals and varies the attenuation of the power of optical signals in order to equalize them. These suffer from poor spectral uniformity/flatness, poor accuracy, long response time, are typically bulky, and are made up of components which are not compactly integrated. Due to the use of electronics for control, often there is a long development life cycle, additional required man-power, and various extra associated costs.
Examples of some of the current solutions are planar lightwave circuits (PLCs) using thermally controlled eVOAs (electronic variable optical attenuators) and AWGs (arrayed waveguide gratings), Mach-Zehnder Interferometers, and MEMs (micro-electrical mechanical switches). Other solutions are free spaced optics based and usually take a liquid crystal approach.
All of these solutions essentially take the approach of attenuating the optical signals, measuring the power of the transmitted signals and in a control loop varying the attenuation to achieve equalization. If the power of the signals were to drift, the complex electronics would vary the attenuation of the signals to a new configuration so that equalization could be achieved. This has inherent response time, and accuracy problems. The response time will be a function of complex electronics used to measure the signal power and logically control the attenuation in a feedback loop until a desired measured power is achieved. In terms of accuracy, the accuracy of equalization does not only depend upon accuracy and calibration of the equalizing elements of the apparatus but also depends upon the accuracy and calibration of the measuring elements of the apparatus. The limitations of the measuring elements, in the closed loop design contribute to the inaccuracy and non-uniformity of equalization. Due in large part to the complex electronics involved in the closed loop design integration on a PLC is not feasible and the result is often bulky, expensive to develop, construct and maintain.
It would be desirable if a channel power equalizer were provided that did not use close-loop control requiring complex electronics, that allowed integration of higher complex functions on a Photonic Lightwave Circuit (PLC), that provided better optical performance such as spectral uniformity, accuracy, and response time, and that was considerably more compact in size.
The present invention provides a dynamic gain equalizer of an open-loop design using nonlinear (NL) optical materials for equalizing channel power. The invention achieves restoration of spectral power uniformity by employing nonlinear optical limiters with desirable power transfer function curves to each of the optical signals to be equalized. Pulse reshaping is provided at no extra cost by power transfer curves which provide a limit transmission power and preferably a steeper region where the curve has a slope of greater than one, which respectively serve to clip pulses at a maximum power and correct the sides of the pulses by stretching. The invention combines the highly desirable functions of dynamic gain equalization, optical pulse reshaping, and in some embodiments noise reduction. Some embodiments constructed according to the invention provide signal dynamic range control by biasing the nonlinear optical limiter with a biasing optical signal.
According to a first broad aspect, the invention provides for an apparatus for equalizing channel powers of a multichannel optical signal having an optical demultiplexer for demultiplexing the multichannel optical signal into a plurality of single channel optical signals, for each single channel optical signal a respective nonlinear optical limiter which is designed to limit the single channel optical signal to produce a limited single channel optical signal, and an optical multiplexer for multiplexing the limited single channel optical signals to produce an equalized multichannel optical signal.
In some embodiments of the invention, each nonlinear optical limiter has a limit transmission power such that the limited single channel optical signal is limited to a power less than or equal to the limit transmission power.
In some embodiments of the invention, the limit transmission powers of the nonlinear optical limiters are equal.
In some embodiments of the invention, each nonlinear optical limiter is designed to produce a limited single channel optical signal according to an optical limiting power transfer curve applied to the respective single channel optical signal, the optical limiting power transfer curve providing a piecewise increasing monotonic transmitted power function portion when incident light upon the nonlinear optical limiter has a power less than an incident light critical power, and providing a relatively flat transmitted power function portion when incident light upon the nonlinear optical limiter has a power greater than the incident light critical power, and in which the limit transmission powers of the nonlinear optical limiters are defined by said relatively flat transmitted power function portion.
Some embodiments of the invention provide for the piecewise increasing monotonic transmitted power function portion having a steeper transmitted power function portion having a slope of greater than one by which sides of optical pulses of the respective single channel optical signal are corrected.
Some embodiments of the invention provide for the piecewise increasing monotonic transmitted power function portion having a transmitted power function portion which limits the power of the respective single channel optical signal to an insignificant transmission power for incident light upon the nonlinear optical limiter having a power less than an incident light power threshold, wherein the incident light power threshold is less than said incident light critical power.
Some embodiments of the invention provide for an amplifier for amplifying the multichannel optical signal.
In some embodiments of the invention, the optical demultiplexer is adapted to amplify the multichannel optical signal.
Some embodiments of the invention provide for each single channel optical signal a respective bias optical signal source providing to the nonlinear optical limiter a respective bias optical signal of a wavelength different from leach of the single channel optical signals, each bias optical signal having a power, each limited single channel optical signal having a power which has a dynamic range, in which the power of each bias optical signal controls the dynamic range of the power of the respective limited single channel optical signal produced by the respective nonlinear optical limiter.
Some embodiments of the invention provide for each single channel optical signal a respective optical combiner in which each optical combiner combines the respective single channel optical signal with the respective bias optical signal before they are input into the respective nonlinear optical limiter.
Some embodiments of the invention provide for an isolator adapted to absorb any power of the single channel optical signals which are reflected from the respective nonlinear optical limiter. Other embodiments of the invention provide for each single channel optical signal a respective isolator adapted to absorb any power of the single channel optical signals which are reflected from the respective nonlinear optical limiter.
In some embodiments of the invention, the nonlinear optical limiters are absorptive nonlinear optical limiters.
In some embodiments of the invention, the nonlinear optical limiters are Bragg gratings comprising nonlinear Kerr materials.
Some embodiments of the invention provide for an equalization analyzer and a bias power controller in which the equalization analyzer determines a respective power measurement for each limited single channel optical signal, the bias power controller controlling the power of each bias optical signal as a function of the power measurements.
According a second broad aspect the invention provides for an apparatus for equalizing channel powers of a multichannel optical signal having an optical demultiplexer for demultiplexing the multichannel optical signal into a plurality of single channel optical signals, a broadband nonlinear optical limiter having a respective separate spatial area for each single channel optical signal, the respective separate spatial area adapted to limit the single channel optical signal to produce a limited single channel optical signal, and an optical multiplexer for multiplexing the limited single channel optical signals to produce an equalized multichannel optical signal.
In some embodiments of the invention, each separate spatial area has a limit transmission power such that the limited single channel optical signal is limited to a power less than or equal to the limit transmission power.
In some embodiments of the invention, the limit transmission powers of the separate spatial areas are equal.
Some embodiments of the invention provide for an amplifier for amplifying the multichannel optical signal.
Some embodiments of the invention provide for an isolator adapted to absorb any power of the single channel optical signals which are reflected from the broadband nonlinear optical limiter.
In some embodiments of the invention, the broadband nonlinear optical limiter is a broadband Bragg grating comprising nonlinear Kerr materials.
According to a third broad aspect, the invention provides for a method of equalizing channel powers of a multichannel optical signal including demultiplexing the multichannel optical signal into a plurality of single channel optical signals, for each single channel optical signal, producing a limited single channel optical signal using a respective nonlinear optical limiter which is adapted to limit the single channel optical signal, and multiplexing the limited single channel optical signals to produce an equalized multichannel optical signal.
Some embodiments of the invention provide for amplifying the multichannel optical signal, before the step of demultiplexing.
Some embodiments of the invention provide for each single channel optical signal, providing to the respective nonlinear optical limiter a respective bias optical signal of a wavelength different from each of the single channel optical signals, each bias optical signal having a power, each limited single channel optical signal having a power which has a dynamic range, and controlling the power of each bias optical signal to control the dynamic range of the power of the respective limited single channel optical signal.
Some embodiments of the invention provide for combining the respective single channel optical signal with the respective bias optical signal before producing the limited single channel optical signal.
Some embodiments of the invention provide for absorbing any power of the single channel optical signals which are reflected from the nonlinear optical limiters.
Some embodiments of the invention provide for determining a respective power measurement for each limited single channel optical signal, and controlling the power of each bias optical signal as a function of the power measurements.
According to a fourth broad aspect, the invention provides for a method of equalizing channel powers of a multichannel optical signal including demultiplexing the multichannel optical signal into a plurality of single channel optical signals, for each single channel optical signal, producing a limited single channel optical signal using a broadband nonlinear optical limiter having a respective separate spatial area for each single channel optical signal, the respective separate spatial area adapted to limit the single channel optical signal, and multiplexing the limited single channel optical signals to produce an equalized multichannel optical signal.
Some embodiments of the invention provide for absorbing any power of the single channel optical signals which are reflected from the broadband nonlinear optical limiter.
Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.